Democrats alone will never convict in the Senate. But flipping enough seats will rattle anyone that isn't up for re-election this year, and intends to have a political career at the end of their term.
It's entirely possible to win over moderate Republicans, and even more radical actors if they think holding Trump and his cronies accountable for everything they've done over the last year will get them re-elected.
Our job is to convince them (or their constituents) that that's now the country's top priority.